Dehydrogenation process using layered catalyst composition

ABSTRACT

This invention relates to a dehydrogenation process using a layered catalyst composition. The catalyst composition comprises an inner core such as alpha-alumina, and an outer layer bonded to the inner core composed of an outer refractory inorganic oxide such as gamma-alumina. The outer layer has uniformly dispersed thereon a platinum group metal such as platinum and a promoter metal such as tin. The composition also contains a modifier metal such as lithium. The catalyst composition shows improved durability and selectivity for dehydrogenating hydrocarbons, especially at dehydrogenation conditions comprising a low water concentration.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of prior U.S. applicationSer. No. 09/887,229, filed Jun. 22, 2001 now U.S. Pat. No. 6,486,370,which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

This invention relates to a hydrocarbon dehydrogenation process using alayered catalyst composition at select operating conditions forincreased catalyst stability.

BACKGROUND OF THE INVENTION

Platinum based catalysts are used for numerous hydrocarbon conversionprocesses. In many applications promoters and modifiers are also used.One such hydrocarbon conversion process is the dehydrogenation ofhydrocarbons, particularly alkanes such as isobutane which are convertedto isobutylene. For example, U.S. Pat. No. 3,878,131 (and related U.S.Pat. Nos. 3,632,503 and 3,755,481) discloses a catalyst comprising aplatinum metal, a tin oxide component and a germanium oxide component.All components are uniformly dispersed throughout the alumina support.U.S. Pat. No. 3,761,531 (and related U.S. Pat. No. 3,682,838) disclosesa catalytic composite comprising a platinum group component, a Group IVA(IUPAC 14) metallic component, e.g., germanium, a Group VA (IUPAC 15)metallic component, e.g., arsenic, antimony, and an alkali or alkalineearth component all dispersed on an alumina carrier material. Again allthe components are evenly distributed on the carrier.

U.S. Pat. No. 6,177,381 describes a dehydrogenation process using alayered catalyst composition. Example 7 of U.S. Pat. No. 6,177,381describes testing of Catalysts A, B, E, and F for dehydrogenationactivity using a hydrocarbon feed. A water concentration of 2000 ppmbased on hydrocarbon weight was injected. The deactivation rates ofCatalysts A, B, E, and F were 0.052, 0.032, 0.050, and 0.033° F./hr,respectively.

Although these deactivation rates are relatively low, otherdehydrogenation processes are sought that have even lower deactivationrates.

SUMMARY OF THE INVENTION

An improved dehydrogenation process using a layered catalyst compositionwhich exhibits excellent stability at a critical combination of catalystproperties and operating conditions is disclosed. When the thickness ofthe outer layer of the layered catalyst is in the range of from 40 to150 microns, the loading of the platinum group metal in the entirelayered catalyst is in the range of from about 5 to about 30 gram-moleof the platinum group metal per cubic meter of the entire layeredcatalyst, and the concentration of the platinum group metal in the outerlayer of the layered catalyst is from about 0.026 to about 0.26gram-mole of the platinum group metal per kilogram of the outer layer,then excellent stability results, provided that the amount of waterpassed to the layered catalyst is less than 1000 wt-ppm, and preferablyless than 300 wt-ppm, and more preferably less than 60 wt-ppm based onthe amount of hydrocarbon passed to the layered catalyst. This resultwas unexpected because previously it had been thought that such highamounts of platinum-group metal in the outer layer would adverselyaffect stability. However, it is now recognized that evendehydrogenation processes using catalysts that have relatively highamounts of platinum-group metal in the outer layer can be operated atlow water concentrations and thus achieve excellent catalyst stability.

In addition, the process disclosed exhibits better selectivity thanprocesses of the prior art, in terms of total selectivity to normalolefins.

In a broad embodiment, this invention is a hydrocarbon dehydrogenationprocess comprising contacting a hydrocarbon stream with a layeredcomposition under dehydrogenation conditions to give a dehydrogenatedproduct. The layered composition comprises an inner core and an outerlayer bonded to the inner core. The outer layer comprises an outerrefractory inorganic oxide and has a thickness of from about 40 to about150 microns. The outer layer also has, uniformly dispersed thereon, atleast one platinum group metal and at least one promoter metal, wherethe concentration of the at least one platinum group metal in the outerlayer is from about 0.026 to about 0.26 gram-mole of the platinum groupmetal on an elemental basis per kilogram of the outer layer. The layeredcomposition has a loading of the at least one platinum group metal offrom about 5 to about 30 gram-mole of the platinum group metal on anelemental basis per cubic meter of the layered composition. The layeredcomposition further has dispersed thereon at least one modifier metal.The inner core and the outer refractory inorganic oxide comprisedifferent materials. The dehydrogenation conditions comprise a weight ofwater passed to the layered composition based on the hydrocarbon weightpassed to the layered composition of less than 1000 ppm.

Other objects and embodiments are described in the detailed descriptionof the invention.

INFORMATION DISCLOSURE

U.S. Pat. No. 6,177,381 describes a dehydrogenation process using alayered catalyst composition. The entire teachings of U.S. Pat. No.6,177,381 are incorporated herein by reference.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a dehydrogenation process that useslayered catalyst composition. The layered catalyst composition comprisesan inner core composed of a material which has substantially loweradsorptive capacity for catalytic metal precursors, relative to theouter layer. Some of the inner core materials are also not substantiallypenetrated by liquids, e.g., metals including but not limited toaluminum, titanium and zirconium. Examples of the inner core materialinclude, but are not limited to, refractory inorganic oxides, siliconcarbide and metals. Examples of refractory inorganic oxides includewithout limitation alpha alumina, theta alumina, cordierite, zirconia,titania and mixtures thereof. A preferred inorganic oxide is alphaalumina.

These materials which form the inner core can be formed into a varietyof shapes such as pellets, extrudates, spheres or irregularly shapedparticles although not all materials can be formed into each shape.Preparation of the inner core can be done by means known in the art suchas oil dropping, pressure molding, metal forming, pelletizing,granulation, extrusion, rolling methods and marumerizing. A sphericalinner core is preferred. The inner core whether spherical or not has aneffective diameter of about 0.05 mm to about 5 mm and preferably fromabout 0.8 mm to about 3 mm. For a non-spherical inner core, effectivediameter is defined as the diameter the shaped article would have if itwere molded into a sphere. Once the inner core is prepared, it iscalcined at a temperature of about 400° C. to about 1500° C.

The inner core is now coated with a layer of a refractory inorganicoxide which is different from the inorganic oxide which may be used asthe inner core and will be referred to as the outer refractory inorganicoxide. This outer refractory oxide is one which has good porosity, has asurface area of at least 50 m²/g, and preferably at least 150 m²/g, anapparent bulk density of about 0.2 g/ml to about 1.0 g/ml and is chosenfrom the group consisting of gamma alumina, delta alumina, eta alumina,theta alumina, silica/alumina, zeolites, non-zeolitic molecular sieves(NZMS), titania, zirconia and mixtures thereof. It should be pointed outthat silica/alumina is not a physical mixture of silica and alumina butmeans an acidic and amorphous material that has been cogelled orcoprecipitated. This term is well known in the art, see e.g., U.S. Pat.Nos. 3,909,450; 3,274,124; and 4,988,659, all of which are incorporatedby reference. Examples of zeolites include, but are not limited to,zeolite Y, zeolite X, zeolite L, zeolite beta, ferrierite, MFI,mordenite and erionite. Non-zeolitic molecular sieves (NZMS) are thosemolecular sieves which contain elements other than aluminum and siliconand include silicoaluminophosphates (SAPOs) described in U.S. Pat. No.4,440,871, ELAPOs described in U.S. Pat. No. 4,793,984, MeAPOs describedin U.S. Pat. No. 4,567,029 all of which are incorporated by reference.Preferred refractory inorganic oxides are gamma and eta alumina.

A preferred way of preparing a gamma alumina is by the well-known oildrop method which is described in U.S. Pat. No. 2,620,314 which isincorporated by reference. The oil drop method comprises forming analuminum hydrosol by any of the techniques taught in the art andpreferably by reacting aluminum metal with hydrochloric acid; combiningthe hydrosol with a suitable gelling agent, e.g.,hexamethylenetetraamine; and dropping the resultant mixture into an oilbath maintained at elevated temperatures (about 93° C.). The droplets ofthe mixture remain in the oil bath until they set and form hydrogelspheres. The spheres are then continuously withdrawn from the oil bathand typically subjected to specific aging and drying treatments in oiland ammoniacal solutions to further improve their physicalcharacteristics. The resulting aged and gelled spheres are then washedand dried at a relatively low temperature of about 80° C. to 260° C. andthen calcined at a temperature of about 455° C. to 705° C. for a periodof about 1 to about 20 hours. This treatment effects conversion of thehydrogel to the corresponding crystalline gamma alumina.

The layer is applied by forming a slurry of the outer refractory oxideand then coating the inner core with the slurry by means well known inthe art. Slurries of inorganic oxides can be prepared by means wellknown in the art which usually involve the use of a peptizing agent. Forexample, any of the transitional aluminas can be mixed with water and anacid such as nitric, hydrochloric, or sulfuric to give a slurry.Alternatively, an aluminum sol can be made by for example, dissolvingaluminum metal in hydrochloric acid and then mixing the aluminum solwith the alumina powder.

It is also required that the slurry contain an organic bonding agentwhich aids in the adhesion of the layer material to the inner core.Examples of this organic bonding agent include but are not limited topolyvinyl alcohol (PVA), hydroxy propyl cellulose, methyl cellulose andcarboxy methyl cellulose. The amount of organic bonding agent which isadded to the slurry will vary considerably from about 0.1 wt-% to about3 wt-% of the slurry. How strongly the outer layer is bonded to theinner core can be measured by the amount of layer material lost duringan attrition test, i.e., attrition loss. Loss of the second refractoryoxide by attrition is measured by agitating the catalyst, collecting thefines and calculating an attrition loss. It has been found that by usingan organic bonding agent as described above, the attrition loss is lessthan about 10 wt-% of the outer layer. Finally, the thickness of theouter layer varies from about 40 to about 150 microns. One micron equals10⁶ meter.

Depending on the particle size of the outer refractory inorganic oxide,it may be necessary to mill the slurry in order to reduce the particlesize and simultaneously give a narrower particle size distribution. Thiscan be done by means known in the art such as ball milling for times ofabout 30 minutes to about 5 hours and preferably from about 1.5 to about3 hours. It has been found that using a slurry with a narrow particlesize distribution improves the bonding of the outer layer to the innercore.

Without wishing to be bound by any particular theory, it appears thatbonding agents such as PVA aid in making an interlocking bond betweenthe outer layer material and the inner core. Whether this occurs by thePVA reducing the surface tension of the core or by some other mechanismis not clear. What is clear is that a considerable reduction in loss ofthe outer layer by attrition is observed.

The slurry may also contain an inorganic bonding agent selected from analumina bonding agent, a silica bonding agent or mixtures thereof.Examples of silica bonding agents include silica sol and silica gel,while examples of alumina bonding agents include alumina sol, boehmiteand aluminum nitrate. The inorganic bonding agents are converted toalumina or silica in the finished composition. The amount of inorganicbonding agent varies from about 2 to about 15 wt-% as the oxide, andbased on the weight of the slurry.

Coating of the inner core with the slurry can be accomplished by meanssuch as rolling, dipping, spraying, etc. One preferred techniqueinvolves using a fixed fluidized bed of inner core particles andspraying the slurry into the bed to coat the particles evenly. Thethickness of the layer can vary considerably, but usually is from about40 to about 150 microns. It should be pointed out that the optimum layerthickness depends on the choice of the outer refractory oxide. Once theinner core is coated with the layer of outer refractory inorganic oxide,the resultant layered support is dried at a temperature of about 100° C.to about 320° C. for a time of about 1 to about 24 hours and thencalcined at a temperature of about 400° C. to about 900° C. for a timeof about 0.5 to about 10 hours to effectively bond the outer layer tothe inner core and provide a layered catalyst support. Of course, thedrying and calcining steps can be combined into one step.

When the inner core is composed of a refractory inorganic oxide (innerrefractory oxide), it is necessary that the outer refractory inorganicoxide be different from the inner refractory oxide. Additionally, it isrequired that the inner refractory inorganic oxide have a substantiallylower adsorptive capacity for catalytic metal precursors relative to theouter refractory inorganic oxide.

Having obtained the layered catalyst support, catalytic metals can bedispersed on the layered support by means known in the art. Thus, aplatinum group metal, a promoter metal and a modifier metal can bedispersed on the outer layer. The platinum group metals includeplatinum, palladium, rhodium, iridium, ruthenium and osmium. Promotermetals are selected from the group consisting of tin, germanium,rhenium, gallium, bismuth, lead, indium, cerium, zinc and mixturesthereof, while modifier metals are selected from the group consisting ofalkali metals, alkaline earth metals and mixtures thereof.

These catalytic metal components can be deposited on the layered supportin any suitable manner known in the art. One method involvesimpregnating the layered support with a solution (preferably aqueous) ofa decomposable compound of the metal or metals. By decomposable is meantthat upon heating the metal compound is converted to the metal or metaloxide with the release of byproducts. Illustrative of the decomposablecompounds of the platinum group metals are chloroplatinic acid, ammoniumchloroplatinate, bromoplatinic acid, dinitrodiamino platinum, sodiumtetranitroplatinate, rhodium trichoride, hexa-amminerhodium chloride,rhodium carbonylchloride, sodium hexanitrorhodate, chloropalladic acid,palladium chloride, palladium nitrate, diamminepalladium hydroxide,tetraamminepalladium chloride, hexachloroiridate (IV) acid,hexachloroiridate (III) acid, ammonium hexachloroiridate (III), ammoniumaquohexachloroiridate (IV), ruthenium tetrachloride,hexachlororuthenate, hexa-ammineruthenium chloride, osmium trichlorideand ammonium osmium chloride. Illustrative of the decomposable promotermetal compounds are the halide salts of the promoter metals. A preferredpromoter is tin and preferred decomposable compounds are stannouschloride or stannic chloride.

The alkali and alkaline earth metals which can be used as modifiermetals in the practice of this invention include lithium, sodium,potassium, cesium, rubidium, beryllium, magnesium, calcium, strontiumand barium. Preferred modifier metals are lithium, potassium, sodium andcesium with lithium and sodium being especially preferred. Illustrativeof the decomposable compounds of the alkali and alkaline earth metalsare the halide, nitrate, carbonate or hydroxide compounds, e.g.,potassium hydroxide, lithium nitrate.

All three types of metals can be impregnated using one common solutionor they can be sequentially impregnated in any order, but notnecessarily with equivalent results. A preferred impregnation procedureinvolves the use of a steam-jacketed rotary dryer. The support isimmersed in the impregnating solution containing the desired metalcompound contained in the dryer and the support is tumbled therein bythe rotating motion of the dryer. Evaporation of the solution in contactwith the tumbling support is expedited by applying steam to the dryerjacket. The resultant composite is allowed to dry under ambienttemperature conditions, or dried at a temperature of about 80° C. toabout 350° C., followed by calcination at a temperature of about 200° C.to about 700° C. for a time of about 1 to about 4 hours, therebyconverting the metal compound to the metal or metal oxide. It should bepointed out that for the platinum group metal compound, it is preferredto carry out the calcination at a temperature of about 400° C. to about700° C.

In one method of preparation, the promoter metal is first deposited ontothe layered support and calcined as described above and then themodifier metal and platinum group metal are simultaneously dispersedonto the layered support by using an aqueous solution which contains acompound of the modifier metal and a compound of the platinum groupmetal. The support is impregnated with the solution as described aboveand then calcined at a temperature of about 400° C. to about 700° C. fora time of about 1 to about 4 hours.

An alternative method of preparation involves adding one or more of themetal components to the outer refractory oxide prior to applying it as alayer onto the inner core. For example, a decomposable salt of thepromoter metal, e.g., tin (IV) chloride can be added to a slurrycomposed of gamma alumina and aluminum sol. Further, either the modifiermetal or the platinum group metal or both can be added to the slurry.Thus, in one method, all three catalytic metals are deposited onto theouter refractory oxide prior to depositing the second refractory oxideas a layer onto the inner core. Again, the three types of catalyticmetals can be deposited onto the outer refractory oxide powder in anyorder although not necessarily with equivalent results.

Another method of preparation involves first impregnating the promotermetal onto the outer refractory oxide and calcining as described above.Next, a slurry is prepared (as described above) using the outerrefractory oxide containing the promoter metal and applied to the innercore by means described above. Finally, the modifier metal and platinumgroup metal are simultaneously impregnated onto the layered compositionwhich contains the promoter metal and calcined as described above togive the desired layered catalyst.

As a final step in the preparation of the layered catalyst composition,the catalyst composition is reduced under hydrogen or other reducingatmosphere in order to ensure that the platinum group metal component isin the metallic state (zero valent). Reduction is carried out at atemperature of about 100° C. to about 650° C. for a time of about 0.5 toabout 10 hours in a reducing environment, preferably dry hydrogen. Thestate of the promoter and modifier metals can be metallic (zero valent),metal oxide or metal oxychloride.

The layered catalyst composition can also contain a halogen componentwhich can be fluorine, chlorine, bromine, iodine or mixtures thereofwith chlorine and bromine preferred. This halogen component is presentin an amount of 0.03 to about 1.5 wt-% with respect to the weight of theentire catalyst composition. The halogen component can be applied bymeans well known in the art and can be done at any point during thepreparation of the catalyst composition although not necessarily withequivalent results. It is preferred to add the halogen component afterall the catalytic components have been added either before or aftertreatment with hydrogen.

Although in the preferred embodiments all three metals are uniformlydistributed throughout the outer layer of outer refractory oxide andsubstantially present only in the outer layer, it is also within thebounds of this invention that the modifier metal can be present both inthe outer layer and the inner core. This is owing to the fact that themodifier metal can migrate to the inner core, when the core is otherthan a metallic core.

Although the concentration of each metal component can varysubstantially, it is desirable that the platinum group metal be presentin a concentration of about 0.01 to about 5 weight percent on anelemental basis of the entire weight of the catalyst and preferably fromabout 0.05 to about 2.0 wt-%. The promoter metal is present in an amountfrom about 0.05 to about 10 wt-% of the entire catalyst while themodifier metal is present in an amount from about 0.1 to about 5 wt-% ofthe entire catalyst. Finally, the atomic ratio of the platinum groupmetal to promoter metal varies from about 0.05 to about 5. In particularwhen the promoter metal is tin, the atomic ratio is from about 0.1:1 toabout 5:1 and preferably from about 0.5:1 to about 3:1. When thepromoter metal is germanium the ratio is from about 0.25:1 to about 5:1and when the promoter metal is rhenium, the ratio is from about 0.05:1to about 2.75:1.

In addition, the layered catalyst for use in the process of thisinvention has a critical concentration of the platinum group metal inthe outer layer. This concentration is generally from about 0.026 toabout 0.26 gram-mole of the platinum group metal, on an elemental basisper kilogram of the outer layer. When the platinum group metal isplatinum, this concentration is from about 0.5 to about 5 wt-% ofplatinum on an elemental basis and based on the weight of the outerlayer. For a given concentration of the platinum group metal in theouter layer, there is a preferred atomic ratio of the platinum groupmetal to the promoter metal. For example, when the platinumconcentration is between about 0.5 and about 3 wt-% of platinum on anelemental basis and based on the weight of the outer layer, thepreferred atomic ratio of platinum to tin is from between about 0.6:1 toabout 1.3:1, increasing as the platinum concentration increases.

Suitable catalysts generally have a loading of the platinum group metalof from about 5 to about 30 gram-mole of the platinum group metal on anelemental basis per cubic meter of the layered catalyst. When theplatinum group metal is platinum, this loading is from about 0.0010 toabout 0.0060 gram of platinum on an elemental basis per cubic centimeterof catalyst.

The concentration of the platinum-group metal in the outer layer can bereadily determined in at least three ways. First, the concentration canbe computed based on the weight of the ingredients used in preparing thelayered catalyst. Second, in the case where the layered catalyst haspreviously been prepared and the inner refractory inorganic oxide isdifferent from the outer refractory inorganic oxide, then the innerlayer refractory inorganic oxide can be separated from the outerrefractory inorganic oxide, and the platinum group metal can beseparately recovered, by known chemical and/or mechanical methods. Then,the concentration of the weight of the platinum group metal can bedetermined from the weight of recovered platinum group metal and theweight of recovered inner refractory inorganic oxide. Finally, energydispersive x-ray spectroscopy or wavelength dispersive spectroscopy(EPMA) using a scanning electron microscope of a sample of the layeredcatalyst may also be used.

Having obtained the layered catalyst, it can be used in a hydrocarbondehydrogenation process. It is critical that the process of thisinvention be carried out at certain conditions in order to achieve thesurprising benefit of improved catalyst stability.

Dehydrogenatable hydrocarbons are contacted with the catalyst of theinstant invention in a dehydrogenation zone maintained atdehydrogenation conditions. This contacting can be accomplished in afixed catalyst bed system, a moving catalyst bed system, a fluidized bedsystem, etc., or in a batch-type operation. A fixed bed system ispreferred. In this fixed bed system the hydrocarbon feed stream ispreheated to the desired reaction temperature and then flowed into thedehydrogenation zone containing a fixed bed of the catalyst. Thedehydrogenation zone may itself comprise one or more separate reactionzones with heating means there between to ensure that the desiredreaction temperature can be maintained at the entrance to each reactionzone. The hydrocarbon may be contacted with the catalyst bed in eitherupward, downward or radial flow fashion. Radial flow of the hydrocarbonthrough the catalyst bed is preferred. The hydrocarbon may be in theliquid phase, a mixed vapor-liquid phase or the vapor phase when itcontacts the catalyst. Preferably, it is in the vapor phase.

Hydrocarbons which can be dehydrogenated include hydrocarbons with 2 to30 or more carbon atoms including normal paraffins, isoparaffins,alkylaromatics, naphthenes and olefins. A preferred group ofhydrocarbons is the group of normal paraffins with 2 to about 30 carbonatoms. Especially preferred normal paraffins are those having 9 to 16carbon atoms. Other especially preferred paraffins are monomethylparaffins and dimethyl paraffins having from 9 to 16 carbon atoms. Eachof the aforementioned hydrocarbons may be present alone or in a mixturewith one or more of any of the other aforementioned hydrocarbons.

Dehydrogenation conditions include a temperature of from about 400° C.to about 900° C., a pressure of from about 1 to about 1013 kPa and aliquid hourly space velocity (LHSV) of from about 0.1 to about 100 hr⁻¹.As used herein, the abbreviation ‘LHSV’ means liquid hourly spacevelocity, which is defined as the volumetric flow rate of liquid perhour divided by the catalyst volume, where the liquid volume and thecatalyst volume are in the same volumetric units. Generally forparaffins, the lower the molecular weight, the higher is the temperaturerequired for comparable conversion. The pressure in the dehydrogenationzone is maintained as low as practicable, consistent with equipmentlimitations, to maximize the chemical equilibrium advantages.

The effluent stream from the dehydrogenation zone generally will containunconverted dehydrogenatable hydrocarbons, hydrogen and the products ofdehydrogenation reactions. This effluent stream is typically cooled andpassed to a hydrogen separation zone to separate a hydrogen-rich vaporphase from a hydrocarbon-rich liquid phase. Generally, thehydrocarbon-rich liquid phase is further separated by means of either asuitable selective adsorbent, a selective solvent, a selective reactionor reactions or by means of a suitable fractionation scheme. Unconverteddehydrogenatable hydrocarbons are recovered and may be recycled to thedehydrogenation zone. Products of the dehydrogenation reactions arerecovered as final products or as intermediate products in thepreparation of other compounds.

The dehydrogenatable hydrocarbons may be admixed with a diluent materialbefore, while or after being flowed to the dehydrogenation zone. Thediluent material may be hydrogen, steam, methane, ethane, carbondioxide, nitrogen, argon and the like or a mixture thereof. Hydrogen isthe preferred diluent. Ordinarily, when hydrogen is utilized as thediluent it is utilized in amounts sufficient to ensure a hydrogen tohydrocarbon mole ratio of about 0.1:1 to about 40:1, with best resultsbeing obtained when the mole ratio range is about 1:1 to about 10:1. Thediluent hydrogen stream passed to the dehydrogenation zone willtypically be recycled hydrogen separated from the effluent from thedehydrogenation zone in the hydrogen separation zone.

Water or a material which decomposes at dehydrogenation conditions toform water such as an alcohol, aldehyde, ether or ketone, for example,may be added to the dehydrogenation zone, either continuously orintermittently, in an amount to provide, calculated on the basis ofequivalent water, less than about 1000 weight ppm of the hydrocarbonfeed stream, preferably less than 300 weight ppm, more preferably lessthan 60 weight ppm, and possibly even less than 1 weight ppm. Theprocess of this invention may be operated with no water or materialwhich decomposes to form water added to the dehydrogenation zone.

The following examples are presented in illustration of this inventionand are not intended as undue limitations on the generally broad scopeof the invention as set out in the appended claims.

EXAMPLE 1

A catalyst was prepared essentially according to the method described inExample 1 of U.S. application Ser. No. 09/887,229, except on a differentscale.

Alumina spheres were prepared by the well-known oil drop method, whichis described in U.S. Pat. No. 2,620,314 which is incorporated byreference. This process involves forming an aluminum hydrosol bydissolving aluminum in hydrochloric acid. Hexamethylene tetraamine wasadded to the sol to gel the sol into spheres when dispersed as dropletsinto an oil bath maintained at about 93° C. The droplets remained in theoil bath until they set and formed hydrogel spheres. After the sphereswere removed from the hot oil, they were pressure-aged at about 135° C.and washed with dilute ammonium hydroxide solution, dried at about 110°C. and calcined at about 650° C. for about 2 hours to give gamma aluminaspheres. The calcined alumina was then crushed into a fine powder havinga particle size of less than 200 microns.

Next, a slurry was prepared by mixing aluminum sol (15 wt-% Al₂O₃) anddeionized water and agitated to uniformly distribute the tin component.To this mixture there were added the above prepared alumina powder and a50% aqueous solution of tin(IV) chloride, and the slurry was ball milledfor approximately 240 minutes thereby reducing the maximum particle sizeto less than 50 microns. This slurry was sprayed onto cordierite coreshaving an average diameter of about 1.8 mm by using a granulating andcoating apparatus to give an outer layer of about 120 microns. At theend of the process, some of the slurry was left which did not coat thecores. This layered spherical support was dried at about 200° C. andthen calcined at about 600° C. in order to convert the pseudoboehmite inthe outer layer into gamma alumina and convert the tin chloride to tinoxide.

The calcined layered support was impregnated with lithium and platinumusing a rotary impregnator by contacting the support with an aqueoussolution (1:1 solution:support volume ratio) containing lithium chlorideand chloroplatinic acid based on support weight. The impregnatedcomposite was heated using a rotary impregnator until no solutionremained, dried at about 315° C. and calcined at about 540° C. andreduced in hydrogen at about 500° C.

The resulting catalyst prepared in this example contained 0.158 wt-%platinum, 0.15 wt-% tin, and 0.22 wt-% lithium with respect to theentire catalyst. This catalyst was identified as catalyst A. Theproperties of catalyst A are summarized in Table 1. The tinconcentration in the layer was computed from the tin content of theslurry. The platinum concentration in the layer was computed bymultiplying the tin concentration in the layer by the weight ratio ofplatinum to tin with respect to the entire catalyst.

EXAMPLE 2

A catalyst was prepared essentially according to the method described inExample 2 of U.S. application Ser. No. 09/887,229, except on a differentscale. The impregnated composite was dried at 315° C. and calcined at540° C. The resulting catalyst prepared in this example had a layerthickness of about 55 micron, and contained 0.553 wt-% platinum, 0.32wt-% tin and 0.14 wt-% lithium. This catalyst was identified as catalystB. The properties of catalyst B are summarized in Table 1.

TABLE 1 Catalyst Identification A B Core Cordierite Cordierite CoreDiameter (in) 0.0673 0.0618 Layer Thickness (micron) 120 55 TinConcentration in Layer 0.65 1.97 (wt-% tin) Platinum Concentration inLayer 0.68 3.4 (wt-% platinum) Platinum Concentration in Layer 0.0350.17 (g-mole platinum per kilogram layer) Platinum Loading 0.012 0.043(g platinum per 10 cc catalyst) Platinum Loading 6.1 22.0 (g-moleplatinum per cubic meter catalyst) Atomic Ratio of Platinum to Tin 0.641.05

EXAMPLE 3

Catalyst B was tested for dehydrogenation activity in a laboratory scaleplant. In a 1.27 cm (½″) reactor, 10 cc of catalyst was placed and ahydrocarbon feed composed of 8.8-9.3 wt-% n-C₁₀, 40.0-41.8 wt-% n-C₁₁,38.6 wt-% n-C₁₂, 8.6-10.8 wt-% n-C₁₃, 0.3-0.8 wt-% n-C₁₄ and 1-1.4 wt-%non-normals was flowed over the catalyst under a pressure of 138 kPa(g)(20 psi(g)), a H₂:hydrocarbon molar ratio of 6:1, and a liquid hourlyspace velocity (LHSV) of 20 hr⁻¹. The total normal olefin concentrationin the product (% TNO) was maintained at 15 wt-% by adjusting reactortemperature.

Recycle hydrogen and hydrocarbon feed were combined upstream of thereactor to form a combined feed, and the combined feed was vaporizedprior to entering the reactor. In this example, the catalyst was testedat water concentrations of 2000, 300, and 40-60 wt-ppm based on theweight of the hydrocarbon in the combined feed. In order to achieve thedesired water concentration of 2000 wt-ppm the recycle hydrogen wasrouted through a water bath, and the temperature and pressure of thewater were set as needed. In order to achieve the desired waterconcentrations of 40-60 and 300 wt-ppm an alcohol that readilydecomposes at dehydrogenation conditions was injected into the combinedfeed. If the water concentration of the recycle hydrogen or hydrocarbonfeed was too high to achieve any of these water concentrations, therecycle hydrogen or hydrocarbon feed was dried by routing through amolecular sieve. The results of the testing are presented in the Table 2below. What is presented is the deactivation rate (slope) which isobtained by plotting temperature (° F.) needed to maintain 15% TNO(total normal olefins) versus time. The data show a substantial decreasein catalyst deactivation rate when operating at water levels of 300wt-ppm or below.

EXAMPLE 4

Catalyst A was tested in the manner described in Example 3 at waterconcentrations of 2000 and 60 wt-ppm based on the weight of thehydrocarbon in the combined feed. The results are shown in the Table 2below. Catalyst A also shows a significant decrease in deactivation ratewhen operating at a water level of 60 wt-ppm. However this benefit isnot as substantial as the benefit seen in Example 3.

EXAMPLE 5

Catalyst A was tested in the manner described in Example 3, but withmodified operating conditions. While the liquid hourly space velocity(LHSV) remained 20 hr⁻¹, the H₂:hydrocarbon molar ratio was dropped to4:1. Water concentrations of 3000 and 1000 wt-ppm based on thehydrocarbon in the combined feed were tested. These water concentrationswere achieved by routing the recycle hydrogen through a water bath andcontrolling the temperature and pressure of the bath. The results areshown in the Table 2 below.

EXAMPLE 6

Catalyst A was tested in the manner described in Example 5, but with aliquid hourly space velocity (LHSV) of 30 hr⁻¹ and a H₂:hydrocarbonmolar ratio of 8:1. The results are shown in the Table 2 below.

In both Examples 5 and 6, the deactivation rate at 3000 wt-ppm wassimilar to that at 1000 wt-ppm. This implies that there is little or nochange in catalyst stability between 3000 and 1000 wt-ppm waterconcentration. It is only below 1000 wt-ppm that an improvement incatalyst stability (a decrease in deactivation rate) is seen.

TABLE 2 Hydrogen/ Hydrocarbon Water Deactivation Ex- LHSV RatioConcentration Rate ample Catalyst (hr⁻¹) (mol/mol) (wt-ppm) (° F./hr) 3B 20 6 2000 0.092  300 0.018 40-60 0.016 4 A 20 6 2000 0.096  60 0.05  5A 20 4 3000 0.212 1000 0.198 6 A 30 8 3000 0.132 1000 0.142

What is claimed is:
 1. A hydrocarbon dehydrogenation process comprisingcontacting a hydrocarbon stream with a layered composition underdehydrogenation conditions to give a dehydrogenated product, the layeredcomposition comprising an inner core, an outer layer bonded to the innercore, the outer layer comprising an outer refractory inorganic oxide andhaving a thickness of from about 40 to about 150 microns and havinguniformly dispersed thereon at least one platinum group metal and atleast one promoter metal and having a concentration of the at least oneplatinum group metal of from about 0.026 to about 0.26 gram-mole of theplatinum group metal on an elemental basis per kilogram of the outerlayer, the layered composition further having dispersed thereon at leastone modifier metal, the inner core and the outer refractory inorganicoxide being different materials, the layered composition further havinga loading of the at least one platinum group metal of from about 5 toabout 30 gram-mole of the platinum group metal on an elemental basis percubic meter of the layered composition, the dehydrogenation conditionscomprising a weight of water passed to the layered composition of lessthan 1000 ppm based on the hydrocarbon weight passed to the layeredcomposition.
 2. The process of claim 1 wherein the weight of waterpassed to the layered composition is less than 300 ppm.
 3. The processof claim 1 wherein the weight of water passed to the layered compositionis less than 60 ppm.
 4. The process of claim 1 further characterized inthat the dehydrogenation conditions comprise a temperature of about 400to about 900° C. and a pressure of about 1 to about 1013 kPa.
 5. Theprocess of claim 1 wherein the inner core is selected from the groupconsisting of alpha alumina, metals, theta alumina, silicon carbide,cordierite, zirconia, titania and mixtures thereof.
 6. The process ofclaim 1 wherein the outer refractory inorganic oxide is selected fromthe group consisting of gamma alumina, delta alumina, theta alumina,silica/alumina, zeolites, nonzeolitic molecular sieves, titania,zirconia and mixtures thereof.
 7. The process of claim 1 wherein theplatinum group metal is selected from the group consisting of platinum,palladium, rhodium, iridium, ruthenium, osmium and mixtures thereof. 8.The process of claim 1 wherein the promoter metal is selected from thegroup consisting of tin, germanium, rhenium, gallium, bismuth, lead,indium, cerium, zinc and mixtures thereof.
 9. The process of claim 1wherein the modifier metal is selected from the group consisting ofalkali metals, alkaline earth metals and mixtures thereof.
 10. Theprocess of claim 1 wherein the hydrocarbon stream comprises at least oneC₂-C₃₀ hydrocarbon selected from the group consisting of normalparaffins, isoparaffins, alkylaromatics, naphthenes, and olefins. 11.The process of claim 1 wherein the hydrocarbon stream comprises normalparaffins having 2 to 15 carbon atoms.
 12. The process of claim 1wherein the hydrocarbon stream comprises monomethyl paraffins ordimethyl paraffins.
 13. A hydrocarbon dehydrogenation process comprisingcontacting a hydrocarbon stream with a layered composition underdehydrogenation conditions to give a dehydrogenated product, the layeredcomposition comprising an inner core, an outer layer bonded to the innercore, the outer layer comprising an outer refractory inorganic oxide andhaving a thickness of from about 40 to about 150 microns and havinguniformly dispersed thereon platinum and tin and having a concentrationof platinum of from about 0.5 to about 5 wt-% of platinum on anelemental basis and based on the weight of the outer layer, the layeredcomposition further having dispersed thereon lithium, the inner core andthe outer refractory inorganic oxide being different materials, thelayered composition further having a loading of platinum metal of fromabout 0.0010 to about 0.0060 gram of platinum on an elemental basis percubic centimeter of the layered composition, the dehydrogenationconditions comprising a weight of water passed to the layeredcomposition of less than 1000 ppm based on the hydrocarbon weight passedto the layered composition.
 14. The process of claim 13 wherein theweight of water passed to the layered composition is less than 300 ppm.15. The process of claim 13 further characterized in that thedehydrogenation conditions comprise a temperature of about 400° C. toabout 900° C. and a pressure of about 1 kPa to about 1013 kPa.
 16. Theprocess of claim 13 wherein the inner core is selected from the groupconsisting of alpha alumina, metals, theta alumina, silicon carbide,cordierite, zirconia, titania and mixtures thereof.
 17. The process ofclaim 13 wherein the outer refractory inorganic oxide is selected fromthe group consisting of gamma alumina, delta alumina, theta alumina,silica/alumina, zeolites, nonzeolitic molecular sieves, titania,zirconia and mixtures thereof.
 18. The process of claim 13 wherein thehydrocarbon stream comprises a C₉-C₁₅ hydrocarbon selected from thegroup consisting of normal paraffins, monomethyl paraffins, and dimethylparaffins.